Pursuant to comments made by @tjd and @O.R. Mapper in my answer to this question, I will reproduce the note that I made:
The points raised by @tjd and @O.R. Mapper, in the comments to this answer, are solid. Why Leia was in the Tatoo system to begin with depends on a question I could not find an answer to; namely, at what point between Polis Massa/Darknell and Tatooine, did the Tantive IV (Leia's ship) attempt to transmit the now complete Death Star plans to the Liberty? The Death Star plans were not stolen all at once, but as described in Operation Skyhook, in a series of actions that started in AX-235 or Danuta (the canon is contradictory but seems to favor Danuta), then Toprawa. The Tantive IV then moved to Polis Massa and Darknell to receive the last parts of the plans, both of which are due west of Tatooine in the 'south' the of galaxy.
I could not find in the canon where the Liberty and Tantive IV met to attempt to transfer the plans before being intercepted/interrupted by the Immortal. From there the Tantive IV and a rebel detachment moved to the Tatoo sector but their general location was betrayed by U-3PO during the Battle of Tatooine. Their specific location in the system was given away when they tried to activate an uplink station, and then Vader and his contingent moved in, leading to the capture of Leia.
The attempted activation of the uplink station still supports that Leia was again trying to transmit the plans (from an out-of-the-way location), rather than being in the Tatoo system to deliver them (which she wasn't, she was there to recruit Obi-wan). She would not have used the HoloNet as it was controlled by the Empire and she most likely could not have transmitted the plans over any large distance via hyperwave communication due its limitations, so it is reasonable that she would have dropped out of hyperspace in an out-of-the-way place to transmit the plans as securely as she could.
Is there any evidence in the canon to suggest where the Liberty and the Tantive IV met to attempt this transmission?